In automobiles, vibration isolators are arranged between an engine and a vehicle body (or frame). One example of vibration isolator applied as an engine mount is disclosed in Patent Document 1: Japanese Patent Application Laid-open Publication No. 2005-23974, which is in the form of a liquid-type isolator. The vibration isolator disclosed in Patent Document 1 includes a liquid space liquid-tightly sealed from outside by an outer cylinder, an elastic body formed of rubber, and a diaphragm. The liquid space is divided by a partition member into a main liquid chamber with the elastic member forming part of its walls, and an auxiliary liquid chamber with the diaphragm forming part of its walls. The main liquid chamber and the auxiliary liquid chamber are communicated with each other by two orifices, i.e., a shaking orifice and an idling orifice, having respective flow resistance for the liquid, which are different from each other.
In the vibration isolator disclosed in Patent Document 1, furthermore, the partition member is provided with a movable rubber plate that is faced to the mail liquid chamber, and also with a first operating air chamber (negative pressure chamber) that is situated on the outer side of the main liquid chamber with the movable rubber plate therebetween. The first operating air chamber is connected, through an air supply/discharge passage and a first air pipeline both extending through the partition member and the housing (outer cylinder member), with a switching valve for effecting changeover between a negative pressure and an atmospheric pressure. The known vibration isolator further includes a valve mechanism arranged between the bottom wall portion of the outer cylinder member and the diaphragm, for opening or closing a shaking orifice formed in the partition member. The valve mechanism is provided with a second operating air chamber (valve chamber) having partition walls, which is partly formed by a rubber pressure member (movable wall), and a coil spring for urging the pressure member in a predetermined closure direction. The second operating air chamber is connected to the switching valve through a connection port and a second air pipeline both extending through the bottom wall of the outer cylinder member. In this instance, when the pressure member is moved to a closed position under the biasing force of the coil spring, the shaking orifice is closed by a valve member that is formed at the center portion of the diaphragm. On the other hand, when the second operating air chamber is supplied with a negative pressure and the pressure member is moved to its open position against the biasing force of the coil spring, the valve member is moved from the partition member together with the pressure member to open the shaking orifice.